Add Stack::begin to quickcheck test (#21)

* add Stack::begin to quickcheck test

* add note about quickcheck test randomness

src/stream/stack.rs

@@ -584,9 +584,17 @@ mod tests {
             Arbitrary,
             Gen,
         };
+
+        // FIXME: This test isn't very clever about how a
+        // sequence of commands is generated. It's more likely
+        // to come up with a set that fails early than one
+        // that manages to push and pop the stack depth a lot.
+        // We could instead weight commands so we're more likely
+        // to generate something valid with a degree of randomness.
 
         #[derive(Clone, Copy, Debug)]
         enum Command {
+            Begin,
             Primitive,
             MapBegin,
             MapKey,
@@ -600,7 +608,7 @@ mod tests {
 
         impl Arbitrary for Command {
             fn arbitrary<G: Gen>(g: &mut G) -> Command {
-                match g.next_u32() % 9 {
+                match g.next_u32() % 10 {
                     0 => Command::Primitive,
                     1 => Command::MapBegin,
                     2 => Command::MapKey,
@@ -610,6 +618,7 @@ mod tests {
                     6 => Command::SeqElem,
                     7 => Command::SeqEnd,
                     8 => Command::End,
+                    9 => Command::Begin,
                     _ => unreachable!(),
                 }
             }
@@ -621,6 +630,9 @@ mod tests {
 
                 for cmd in cmd {
                     match cmd {
+                        Command::Begin => {
+                            let _ = stack.begin();
+                        },
                         Command::Primitive => {
                             let _ = stack.primitive();
                         },